This invention relates to internal combustion engines of the reciprocating-piston kind, utilising liquid or gaseous hydrocarbons fuel. The invention, which concerns the ignition of the air/fuel mixture in the combustion chamber of the engine, is particularly, although not exclusively, applicable to reciprocating-piston petrol and diesel engines although it may also be applied to engines utilizing other forms of liquid fuel such as methyl or ethyl alcohols and even gaseous fuels.
With the rising number of road vehicles with i.c. engines in regular use, particularly in urban areas, problems of air pollution have become acute, particularly in certain places having long periods of sunshine, due to photo-chemical reactions between the atmosphere and the exhaust emission of such vehicle engines. This effect manifests itself as "smog" due to complexes of nitrogen oxides, giving rise to reduced visibility and irritation of the eyes, nose and throat. This had led to the introduction of legislation in at least one country to limit locally the quantities of unburnt hydrocarbon, carbon monoxide and nitrogen oxides present in the exhaust emissions of all road vehicle engines. This legal limitation has hardened progressively with time, to an extent which poses considerable technical difficulties to vehicle and engine manufacturers.
One approach to the problem of reducing undesirable exhaust emission is to operate engines with as weak a mixture strength as possible, i.e. as high an air/fuel ratio as is practicable, giving a marked reduction in NO.sub.x emission with increases in air/fuel ratio but with an increase in HC at high air/fuel ratios. NO.sub.x is formed at high combustion temperatures and once formed remains very stable. However with high air/fuel ratios the rate of combustion reaction is slowed down and less heat is released from the smaller quantity of fuel used, so that the temperature rise of the whole air/fuel mass is less than with stoichiometric mixtures. On the other hand the actual burning efficiency of the hydrocarbon fuel is lowered at high air/fuel ratios due to its greater diffusion into the air present, so that although it is possible to ignite the fuel locally by an intense spark at weak mixtures, the flame often fails to reach all the fuel present and as a result some unburnt hydrocarbons remain in the exhaust gases.
Numerous attempts have been made to overcome this by designing so-called "stratified charge" petrol engines in which the fuel is concentrated in a locally-rich portion of charge in proximity to the spark plug or ignition point, and this rich mixture burns rapidly on ignition before being dispersed into the remainder of the charge which may consist of air only. Whilst some reduction in unburnt hydrocarbon levels in the exhaust emissions has been obtained with this approach, there is still ample room for improvement.
At present, the only two methods of igniting the air/fuel mixture in an i.c. piston engine employed in practice are spark ignition and compression ignition, each of which relies on the spread of combustion by a flame front throughout the whole of the mixture charge.
An object of the invention is to provide a new and different way of catalytically igniting the charge in an i.c. piston engine, which does not rely on a flame front to spread the combustion throughout the charge, and which will enable very weak mixtures to be utilized which are not burnable by ordinary spark ignition or compression ignition means.
It has been proposed to utilise a catalyst member mounted in the combustion chamber of the or each cylinder of a piston engine to ignite the air/fuel charge and so initiate the combustion. The catalyst member in this proposal takes the form of a relatively thick disc of honeycomb structure fixedly mounted in the cylinder to extend completely across the cylinder bore above the top-dead-centre position of the piston and below the cylinder head, the fuel introduced into the combustion chamber contacting the catalyst and mixing with the air compressed in the cylinder during the preceding compression stroke, so that the air/fuel mixture is ignited by the catalyst and passes through the catalytic disc to complete the combustion.
With such an arrangement, however, the presence of the relatively thick catalyst member in the cylinder bore between the piston crown and the cylinder head is likely, in a practical design of engine, to impose a serious restriction on the possible value of the compression ratio, if mechanical interference is to be avoided. A low compression ratio leads to unacceptably low thermal efficiency and fuel economy. Moreover, since the effect of the use of the catalyst for ignition is to permit oxidation of the fuel, i.e. combustion, at weaker mixture strengths than can be used with conventional forms of ignition, it would be desirable to take advantage of this to utilise higher than normal compression ratios without "knock" in conjunction with air/fuel mixtures specifically weaker than that for stoichiometric mixture strength.